Corporal Adrian Shepard is ready, though he doesn’t know what he’s in for. Shepard is the hero of Half-Life: Opposing Force, which tells the other side of an already excellent story. In the first game, scientist Gordon Freeman must survive Marines sent to "clean" Black Mesa of its occupants.

Shepard is one of those Marines, sent on what they are told is an indoor combat training mission. While many of his cohorts land safely, Shepard is shot down and must work his way through Black Mesa. It’s a harrowing journey, even for a hardened soldier, fraught with collapsing rooms and all-new aliens. And then there’s that insidious Man in Black…

Opposing Force is an excellent trip back through the darkened halls and alien world of Half-Life. It’s still puzzle-based, though one would expect a story about soldiers to be more combat-oriented. There’s plenty of combat, no doubt, with a few new weapons to go along with it. Better yet, OpFor allows for more squad combat: Shepard can recruit other Marines left behind to cover him.

As in sister game Team Fortress, those other soldiers have specialties. Some are straight-ahead fighters; others include medics who heal the player and fellow Marines, and the engineer who can cut through otherwise impassable doors.

They’ll be facing a few new alien foes, from razor-shooting drones to symbiotic shock troops and monstrous lightning-shooting spider-things.

Very impressive are the ties to Half-Life; finally, the player gets to see the confusion on the other side. You’ll get to see some of the places Freeman will later have to traverse, and see in action things only previously heard, like the "paint your targets" instruction.

This add-on has a few shortcomings. The game itself is relatively short, with an even more ambiguous ending than Half-Life. That doesn’t leave much time for building suspense, as the first game did so well. Oh, the player will still think twice before entering a dark hallway, but the lingering dread so carefully established through the first game just doesn’t have time to take hold here.

Enemy AI is excellent, but NPC Marine AI is less so. I had one engineer stand in a firepit, complaining loudly but never moving. Computer-controlled Marines are also just a little too gung-ho to rush into battle, even when wounded. They’re killed off quickly, leaving Shepard to his own devices.

In all, it’s a nice addition to a legendary story, and it definitely leaves players waiting breathlessly for another chapter.